Apparatus and method for removal of surface oxides via fluxless technique involving electron attachment and remote ion generation

ABSTRACT

The present invention provides a method and apparatus for the dry fluxing of at least one component and/or solder surface via electron attachment. In one embodiment, there is provided a method for removing oxides from the surface of a component comprising: providing a component on a substrate wherein the substrate is grounded or has a positive electrical potential to form a target assembly; passing a gas mixture comprising a reducing gas through an ion generator comprising a first and a second electrode; supplying an amount of voltage to at least one of the first and second electrodes sufficient to generate electrons wherein the electrons attach to at least a portion of the reducing gas and form a negatively charged reducing gas; and contacting the target assembly with the negatively charged reducing gas to reduce the oxides on the component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/425,426, filed Apr. 28, 2003, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to fluxless processes forremoving surface oxides. More specifically, the invention relates to anapparatus and method comprising same for fluxless reflow and solderinginvolving electron attachment within a remote ion generator.

Reflow and soldering are important processing steps in the assembly ofelectronic components for making solder joints. The term “reflow” asused herein refers to a process for making a previously applied solderon a substrate melt and flow upon application of an energy source suchas, for example, thermal energy. The term “soldering” as used hereinrefers to a process that allows a melted solder to join at least twometallic substrates. A variety of different reflow and solderingprocesses may be used in the assembly of electronic devices, such as,but not limited to, reflow of solder bumps used for wafer bumping,reflow soldering used in the assembly of surface-mount electroniccomponents, and wave soldering used in the assembly of insertion-mountcomponents.

Reflow soldering is a process used for outer lead bonding ofsurface-mount components wherein a chip is transferred with leads inplace to the next-level surface mount packages. In the reflow solderingprocess, components are mounted on the corresponding trace area of acircuit board with a solder paste previously printed on the circuitboard. Such formed soldering parts are then loaded into a reflow furnaceand passed through heating and cooling zones. Solder joints betweencomponent leads and solder lands on the circuit board are formed bymelting, wetting, and solidifying the solder paste. To ensure a goodwetting of the molten solder on the joining surfaces, organic fluxes arenormally contained in the solder pastes to remove initial surface oxideson both solder and base metal and to keep the surfaces in a clean statebefore solidification. The fluxes are mostly evaporated into vapor phaseduring soldering, however, the flux volatiles may cause problems, suchas forming voids in the solder joints and contaminating the reflowfurnace. After soldering, flux residues still remain on the circuitboard that can cause corrosion and electric shorts.

Wave soldering is also used for outer lead bonding such as forassembling traditional insertion mount components. It also can be usedfor surface-mount components by temporarily bonding the components onthe circuit board by an adhesive before soldering. For both cases, thecircuit boards with components inserted or temporarily bonded have to becleaned by using a liquid flux to remove oxides on the component leadsand solder lands and then passed through a high temperature moltensolder bath. The molten solder automatically wets the metal surfaces tobe soldered and solder joints are thus formed. The molten solder in thebath has a high tendency to be oxidized, forming solder dross. Thereforethe surface of the solder bath has to be frequently cleaned bymechanically removing the dross, which increases the operation cost andthe consumption of the solder. After soldering, flux residues remain onthe circuit boards, which brings the same problems as described hereinfor reflow soldering.

Wafer bumping is a process used to make thick metal bumps on the chipbond pads for inner lead bonding. The bumps are commonly made bydepositing a solder on the pads and then reflowing (referred to hereinas a first reflow) to conduct alloying and to change the shape of thesolder bump from a mushroom-shape into a hemispherical-shape. The chipwith the first-reflowed bumps is “flipped” to correspond to thefootprint of the solder wettable terminals on the substrate and thensubjected to a second reflow to form solder joints. These solder jointsare referred to herein as inner lead bonds. High-melting point solders(e.g., >300° C.) are normally used in the wafer bumping process becauseit allows for subsequent assembly steps such as outer lead bonding toproceed using lower-melting point solders (e.g., <230° C.) withoutdisruption of the inner lead bonds.

The shape of the solder bumps after the first reflow is critical. Forexample, a large bump height is preferable for better bonding and higherfatigue resistance. Further, the bumps formed should preferably besubstantially uniform to ensure planarity. Substantially uniform solderbumps having relatively larger bump heights is believed to be associatedwith an oxide-free bump surface during the first reflow. One approachfor removing solder oxides during the first reflow of the solder bumpedwafer is applying organic fluxes over the deposited solder bumps, orwithin a solder paste mixture that has been printed onto the wafer toform the bumps, and reflowing the bumps in an inert environment so thatthe fluxes can effectively remove initial oxides on the solder surface.However, this approach has its drawbacks. Small voids may form in thesolder bumps due to flux decomposition. These voids may not only degradethe electrical and mechanical properties of the formed solder bonds butalso destroy the co-planarity of the solder bumped wafer and affect thesubsequent chip bonding process. The decomposed flux volatiles can alsocontaminant the reflow furnace which can increase the maintenance cost.In addition, flux residues are oftentimes left upon the wafer which cancause corrosion and degrade the performance of the assembly.

To remove the flux residues from the reflow and soldering processesdescribed above, a post cleaning process may be adopted usingchlorofluorcarbons (CFCs) as cleaning agents. However, post-cleaningadds an additional process step and increases the manufacturingprocessing time. Further, the use of chlorofluorocarbons (CFCs) ascleaning agents is banned due to the potential damage to the earth'sprotective ozone layer. Although no-clean fluxes have been developed byusing a small amount of activators to reduce residues, there is atrade-off between the gain and loss in the amount of flux residues andthe activity of the fluxes.

A good solution to all the problems described above, including voidformation, flux volatiles, flux residues, and dross formation, is usinga reducing gas as a reflow and soldering environment to replace organicfluxes for removing metal oxides. Such reflow and soldering techniquesare called “fluxless reflow” and “fluxless soldering”. Among variousfluxless reflow and soldering methods, the use of hydrogen as a reactivegas to reduce oxides on base metals and solders is especially attractivebecause it is a very clean process (the only by-product is water whichcan be easily ventilated out of the furnace), and it can be compatiblewith an open and continued soldering production line (H₂ is non-toxicand has a flammable range of 4 to 75%). Therefore, hydrogen fluxlesssoldering has been a technical goal for a long time.

One previously used hydrogen fluxless method for inner lead bonding hasbeen to employ pure hydrogen for reflow of the solder bumped wafer attemperatures ranging from 400 to 450° C. However, the flammable natureof the pure hydrogen largely limits its application. For solderingprocesses used in outer lead bonding, such as reflow soldering and wavesoldering, the major limitation of using hydrogen to reduce surfaceoxides is the inefficient and slow reduction rate of metal oxides at thenormal processing temperature ranges (<250° C.), especially for solderoxides, which have higher metal-oxygen bond strengths than that of theoxides on the base metals to be soldered. This inefficiency of hydrogenis attributed to the lack of reactivity of the hydrogen molecule at lowtemperatures. Highly reactive radicals, such as mono-atomic hydrogen,form at temperatures much higher than the normal reflow soldering andwave soldering temperature range. For example, the effective temperaturerange for pure H₂ to reduce tin oxides on a tin-based solder is above350° C. Such high temperatures may either damage or cause reliabilityproblems to the packed electronic components. Therefore, a catalyticmethod to assist generating highly reactive H₂ radicals, and thusreducing the effective ranges of hydrogen concentration and processingtemperature for reducing surface oxides, has been sought by theindustry.

Fluxless (dry) soldering has been performed in the prior art usingseveral techniques. One technique is to employ lasers to ablate or heatmetal oxides to their vaporization temperatures. Such processes aretypically performed under inert or reducing atmospheres to preventre-oxidation by the released contaminants. However, the melting orboiling points of the oxide and base metal can be similar and it may notbe desirable to melt or vaporize the base metal. Therefore, such laserprocesses are difficult to implement. Lasers are typically expensive andinefficient to operate and require a direct line of sight to the oxidelayer. These factors limit the usefulness of laser techniques for mostsoldering applications.

Surface oxides can be chemically reduced (e.g., to H₂O) through exposureto reactive gases (e.g., H₂) at elevated temperatures. A mixturecontaining 5% or greater reducing gas in an inert carrier (e.g., N₂) istypically used. The reaction products (e.g., H₂O) are then released fromthe surface by desorption at the elevated temperature and carried awayin the gas flow field. Typical process temperatures exceed 350° C.However, this process can be slow and ineffective, even at elevatedtemperatures.

The speed and effectiveness of the reduction process can be increasedusing more active reducing species. Such active species can be producedusing conventional plasma techniques.

Gas plasmas at audio, radio, or microwave frequencies can be used toproduce reactive radicals for surface de-oxidation. In such processes,high intensity electromagnetic radiation is used to ionize anddissociate H₂, O₂, SF₆, or other species, including fluorine-containingcompounds, into highly reactive radicals. Surface treatment can beperformed at temperatures below 300° C. However, in order to obtainoptimum conditions for plasma formation, such processes are typicallyperformed under vacuum conditions. Vacuum operations require expensiveequipment and must be performed as a slow, batch process rather than afaster, continuous process. Also, plasmas are typically disperseddiffusely within the process chamber and are difficult to direct at aspecific area. Therefore, the reactive species cannot be efficientlyutilized in the process. Plasmas can also cause damage to processchambers through a sputtering process, and can produce an accumulationof space charge on dielectric surfaces, leading to possiblemicro-circuit damage. Microwaves themselves can also cause micro-circuitdamage, and substrate or component temperature may be difficult tocontrol during treatment. Plasmas can also release potentially dangerousultraviolet light. Such processes also require expensive electricalequipment and consume considerable power, thereby reducing their overallcost effectiveness.

U.S. Pat. No. 5,409,543 discloses a process for producing a reactivehydrogen species (i.e., atomic hydrogen) using a hot filament tothermally dissociate molecular hydrogen in a vacuum condition. Theenergized hydrogen chemically reduces the substrate surface. Thetemperature of the hot filament may range from 500° C. to 2200° C.Electrically biased grids are used to deflect or capture excess freeelectrons emitted from the hot filament. The reactive species or atomichydrogen are produced from mixtures containing 2% to 100% hydrogen in aninert carrier gas.

U.S. Pat. No. 6,203,637 discloses a process for activating hydrogenusing the discharge from a thermionic cathode. Electrons emitted fromthe thermionic cathode create a gas phase discharge which generatesactive species. The emission process is performed in a separate orremote chamber containing a heated filament. Ions and activated neutralsflow into the treatment chamber to chemically reduce the oxidized metalsurface. However, such hot cathode processes require vacuum conditionsfor optimum effectiveness and filament life. Vacuum operations requireexpensive equipment, which must be incorporated into soldering conveyorbelt systems, thereby reducing their overall cost effectiveness.

Potier, et al., “Fluxless Soldering Under Activated Atmosphere atAmbient Pressure”, Surface Mount International Conference, 1995, SanJose, Calif., and U.S. Pat. Nos. 6,146,503, 6,089,445, 6,021,940,6,007,637, 5,941,448, 5,858,312 and 5,722,581 describe processes forproducing activated H₂ (or other reducing gases, such as CH₄ or NH₃)using electrical discharge. The reducing gas is generally present at“percent levels” in an inert carrier gas (N₂). The discharge is producedusing an alternating voltage source of “several kilovolts”. Electronsemitted from electrodes in a remote chamber produce exited or unstablespecies that are substantially free of electrically charged specieswhich are then flowed to the substrate. The resulting processes reduceoxides on the base metal to be soldered at temperatures near 150° C.However, such remote discharge chambers require significant equipmentcosts and are not easily retrofitted to existing soldering conveyor beltsystems. In addition, these processes are typically employed forpre-treating the metal surface before soldering rather than removingsolder oxides.

U.S. Pat. No. 5,433,820 describes a surface treatment process usingelectrical discharge or plasma at atmospheric pressure from a highvoltage (1 kV to 50 kV) electrode. The electrode is placed in theproximity of the substrate rather than in a remote chamber. The freeelectrons emitted from the electrodes produce reactive hydrogenradicals—a plasma containing atomic hydrogen—which then pass throughopenings in a dielectric shield placed over the oxidized substrate. Thedielectric shield concentrates the active hydrogen onto those specificsurface locations requiring de-oxidation. However, such dielectricshields can accumulate surface charge that may alter the electric fieldand inhibit precise process control. The described process is only usedto flux base metal surfaces.

Accordingly, there is a need in the art to provide an economical andefficient process for removing metal oxides from at least one componentand/or solder surface under relatively low temperatures to avoid anydamage to the electronic components. There is a further need in the artto provide a process and apparatus for fluxless soldering under nearambient or atmospheric pressure conditions to avoid the expense ofpurchasing and maintaining vacuum equipment. Additionally, there is anadditional need in the art to provide a fluxless soldering process usinga non-flammable gas environment.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies some, if not all, of the needs of theart by providing a method for removing metal oxides from at least onecomponent and/or solder surface without requiring a flux. Specifically,in one aspect of the present invention, there is provided a method forremoving metal oxides from a surface of at least one componentcomprising: providing at least one component that is connected to asubstrate to form a target assembly wherein the substrate has at leastone electrical potential selected from the group consisting of groundedor positive electrical potential; passing a gas mixture comprising areducing gas through an ion generator comprising a first and a secondelectrode; supplying voltage to at least one of the first and the secondelectrodes sufficient to generate electrons that attach to at least aportion of the reducing gas and form a negatively charged reducing gas;and contacting the target assembly with the negatively charged reducinggas to reduce the oxides on the at least one component.

In another aspect of the present invention, there is provided a methodof fluxless soldering of at least one component to be soldered whereinthe at least one component has surface metal oxides and solder. Themethod comprises: providing the at least one component which isconnected to a substrate as a target assembly wherein the substrate hasat least one electrical potential selected from the group consisting ofgrounded or positive electrical potential; passing a gas mixturecomprising a reducing gas and a carrier gas through an ion generatorcomprising a gas inlet, a gas outlet in fluid communication with the gasinlet, an anode, and a cathode interposed between the gas inlet and thegas outlet wherein the gas outlet is proximal to the target assembly;supplying energy to at least one of the cathode and the anode sufficientto generate electrons which attach to at least a portion of the reducinggas passing through the ion generator thereby forming a negativelycharged reducing gas at the gas outlet; and contacting the targetassembly with the negatively charged reducing gas to reduce the metaloxides on the at least one component.

In a still further aspect of the present invention, there is provided anapparatus generating a negatively charged ionic reducing gas comprising:an enclosure defining an interior hollow wherein at a least a portion ofthe enclosure comprises an anode connected to a first voltage level; agas inlet and a gas outlet that is in fluid communication with theinterior hollow; and a cathode residing within the interior hollow andinterposed between the gas inlet and the gas outlet wherein the cathodeis connected to a second voltage level which has a negative biasrelative to the first voltage level.

In yet another aspect of the present invention, there is provided anapparatus for generating a negatively charged ionic reducing gascomprising: a first chamber and a second chamber. The first chamber hasat least two electrodes contained therein wherein a bias in electricpotential is applied between the two electrodes; a first gas inlet toreceive a reducing gas; and a gas outlet to release the negativelycharged ionic reducing gas. The second chamber encloses the firstchamber and has a second gas inlet to receive a carrier gas wherein thesecond gas inlet is in fluid communication with the first chamber andthe reducing gas and the carrier gas form a gas mixture within thesecond chamber.

These and other aspects of the invention will become apparent from thefollowing detailed description.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate a voltage pulse for a cathode and for ananode, respectively.

FIGS. 2 a through 2 i is a schematic illustration of various electrodedesigns for the present invention.

FIG. 3 provides an example of one embodiment of the emission electrodeemploying a plurality of tips.

FIG. 4 provides an example of one embodiment of the emission electrodehaving a segmented assembly.

FIG. 5 provides an example of one embodiment of the present inventionillustrating remote ion generation.

FIG. 6 provides an example of one embodiment of the ion generatorapparatus of the present invention.

FIG. 7 provides a further example of one embodiment of the ion generatorapparatus of the present invention.

FIG. 8 provides a graph depicting voltage versus current for variouscathode temperatures using one embodiment of the apparatus of thepresent invention.

FIG. 9 shows emission current versus frequency and amplitude of pulsedvoltage applied between two electrodes using one embodiment of theapparatus of the present invention.

FIGS. 10 a, 10 b, 10 c, and 10 d provide an exploded, side view, adetailed view of the anode, and a detailed view of the cathode holderand cathode emitted, respectively, of an additional embodiment of theion generator apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Method and apparatus for the removal of metal oxides from at least onecomponent and/or solder surfaces by exposure to negatively chargedhydrogen ions are disclosed herein. In certain embodiments, the exposuremay occur before and/or during the reflow and soldering processes. Thenegatively charged hydrogen ions react and reduce the surface metaloxides. The present invention can be employed by modifying traditionalreflow and soldering equipments such as, for example, the reflowmachines used for inner lead bonding and the reflow soldering or wavesoldering machines used for outer lead bonding. The present inventioncan also be applied to other processes wherein the removal of thesurface metal oxide is desired such as, but not limited to, metalplating (i.e., the solder plating of portions of printed circuit boardsor metal surfaces to make them more amenable to subsequent soldering),surface cleaning, brazing, welding, and removing surface oxides ofmetals, such as copper oxide, formed during silicon wafer processing.The removal of metal oxides using the method and apparatus of thepresent invention is equally applicable to the aforementioned processesor any other process desirous of removing oxides without the need fororganic fluxes.

The ion generator apparatus disclosed herein may be particularlysuitable for certain embodiments, such as for example, fluxlesssoldering of surface mount components including flip chip assemblies. Inthese embodiments, electrons may penetrate across the component and canaccumulate underneath since the component may be comprised of asemiconductive material. The accumulation of electrons underneath thecomponent can repel the negatively charged active species within thereducing gas thereby effecting the efficiency of the oxide removal. Toremedy this, the position of the ion generator apparatus can be adjusteddepending upon the surface configuration of the component. By using theion generator apparatus disclosed herein, the negatively charged activespecies within the gas mixture can be uniformly distributed on thesurface of the component to be treated regardless of the surfaceconfiguration and can allow for the optimization of operatingconductions (e.g., temperature, pressure, and gas concentration) forforming the negatively charge species without limitation to the surfaceenvironment condition.

The term “component” as used herein generally relates to a componentcomprised of a material such as silicon, silicon coated with silicondioxide, aluminum-aluminum oxide, gallium arsenide, ceramic, quartz,copper, glass, epoxy, or any material suitable for use within anelectronic device. In certain embodiments, the component has solderdisposed upon at least one of its surfaces. Exemplary soldercompositions include, but are not limited to, a fluxless tin-silver, afluxless tin-silver-copper, a fluxless tin-lead, or a fluxlesstin-copper. However, the method of the present invention is suitable fora variety of different components and/or solder compositions.

While not wishing to be bound by theory, it is believed that when anenergy source such as direct current (DC) voltage is applied to at leastone of two electrodes contained within an remote ion generator therebycreating an electrical potential, electrons are generated from anegatively biased electrode, from the gas phase between the twoelectrodes, or a combination thereof and drift toward a positivelybiased electrode along the electric field. In certain preferredembodiments, the component in which the oxide is to be removed and/orsoldered is positioned on a grounded or a positively biased substrateand is disposed within close proximity to the outlet of the remote iongenerator. A gas mixture comprising a reducing gas and optionally acarrier gas is passed through the electric field generated by theelectrodes within the remote ion generator. During the electron drift,part of the reducing gas forms negative ions by electron attachmentwhich then pass through the outlet of the ion generator and attach oradsorb onto the at least one component. The attached or adsorbednegatively charged ions can thus reduce the existing oxides on the basemetal and/or solder without the need for traditional fluxes. In certainpreferred embodiments, the adsorption of the active species on thesurfaces to be treated is promoted due to the opposite electricalcharges between the active species and the surface to be treated (e.g.,target assembly is positively biased).

In embodiments wherein the reducing gas comprises hydrogen, it isbelieved that the method of the present invention occurs as follows:

Dissociative Attachment: H₂ + e′

 H⁻ + H (I) Radiative Attachment: e′ + H

 H⁻ + hy (II) The combination of (I) and (II): 2e′ + H₂

 2H⁻ + hy (III) Oxide Reduction: 2H⁻ + MO

 M + H₂O + 2e′ (IV) (M = solder/base metal)In these embodiments, the activation energy of oxide reduction using theelectron attachment method of the present invention is lower thanmethods that use molecular hydrogen because the formation of atomichydrogen ions with electron attachment eliminates the energy associatedwith bond breaking of molecular hydrogen.

Energy is supplied to at least one of the electrodes preferably thecathode, sufficient to cause electron generation from the cathode, fromthe gas phase between two electrodes, or combinations thereof. Incertain embodiments, the energy source can be an electric energy source,such as an alternating current (AC) or direct current (DC) voltagesource. Other energy sources, such as thermal energy, electromagneticenergy, or photo energy sources may also be used alone or incombination. The energy source may be constant or alternatively pulsed.In certain embodiments of the present invention, the cathode isconnected to a voltage source at a first voltage level and the anode isconnected to a voltage source at a second level. The difference in thevoltage levels creates a bias in electrical potential. One of the firstor second voltage levels may be zero indicating that either the cathodeor the anode is grounded.

To produce negatively charged ions by electron attachment, a largequantity of electrons needs to be generated. In this connection, theelectrons can be generated by a variety of ways such as, but not limitedto, cathode emission, gas discharge, or combinations thereof. Amongthese electron generation methods, the selection of the method dependsmainly on the efficiency and the energy level of the electronsgenerated. For embodiments wherein the reducing gas comprises hydrogen,electrons having an energy level approaching 4 eV may be preferred. Inthese embodiments, such low energy level electrons can be generated bycathode emission and/or gas discharge. The generated electrons may thendrift from the cathode toward the anode which creates a space charge.The space charge provides the electron source for generating thenegatively charged ions when the reducing gas passes through the atleast two electrodes.

For embodiments involving electron generation through cathode emission,these embodiments may include: field emission (referred to herein ascold emission), thermal emission (referred to herein as hot emission),thermal-field emission, photoemission, and electron or ion beamemission.

Field emission involves applying an electric field between the cathodeand the anode that is sufficiently high in intensity to overcome anenergy barrier for electrons to be emitted from the cathode surface. Incertain preferred embodiments, a DC voltage is applied between a cathodewith a large surface curvature and an anode at a voltage ranging from0.1 to 50 kV, or ranging from 1 to 30 kV. In these embodiments, thedistance between the electrodes may range from 0.1 to 30 cm, or from 0.5to 5 cm.

Thermal emission, on the other hand, involves using a high temperatureto energize electrons in the cathode and separate the electrons from themetallic bond in the cathode material. In certain preferred embodiments,the temperature of the cathode may range from 800 to 3500° C., or from800 to 1500° C. The cathode may be brought to and/or maintained at ahigh temperature by a variety of methods such as, but not limited to,directly heating by passing an energy source such as AC or DC currentthrough the cathode; indirect heating such as contacting the cathodesurface with an electrically insulated hot surface heated by a heatingelement, IR radiation, pre-heating the gas mixture to a temperatureequal to or greater than the desired temperature of the cathode; orcombinations thereof.

Thermal-field emission is a hybrid of field emission and thermalemission methods in which both an electric field and a high temperatureare applied. Therefore, thermal-field emission may require a lesserelectric field and a lower cathode temperature for generating the samequantity of electrons as compared with pure field emission and purethermal emission. Thermal-field emission can minimize difficultiesencountered with pure field emission, such as the tendency ofdegradation in electron emission by contamination on the emissionsurface, and a high restriction on the planarity and uniformity of theemission surface. Thermal-field emission may also avoid problems relatedto thermal emission such as a high potential of chemical reactionbetween the emission electrode and gas phase. In embodiments wherein thethermal-field emission is used, the temperature of the cathode can rangefrom ambient to 3500° C., or from 150 to 1500° C. In these embodiments,the electric field can range from 0.01 to 30 KV, or from 0.1 to 10 KV.

In certain preferred embodiments, the thermal emission or thermal-fieldemission mechanism is used for electron generation in reflow orsoldering processes. In these embodiments, the high temperature cathodeused in either of these mechanisms may also act as a heat source for thegas mixture that is passed between the anode and the cathode in the iongenerator. In certain embodiments, the temperature of the gas mixtureinside the ion generator can be at or near the reflow and solderingtemperatures so that the thermal energy required for heating the gas forreflow and soldering can be reduced. In an alternative embodiment, thetemperature of the gas mixture inside the ion generator can berelatively higher than that of the reflow and soldering processes. Inthe latter embodiment, the electron attachment process in some cases,such as when H₂ is used as a reducing gas, can be promoted inside theion generator since the formation of the negatively charge hydrogen ionsby electron attachment is an endothermic reaction. The temperature ofthe negatively charged ionic reducing gas at the outlet of the iongenerator can be reduced to the reflow and soldering temperatures by,for example, diluting the treated gas with bulk furnace gas. As long asthe negatively charged ions are formed, the ions may be repelled fromeach other due to like-charge, thereby reducing the tendency ofrecombination at a reduced temperature.

In certain embodiments of the present invention, the electron generationis accomplished via a combination of cathode emission and coronadischarge methods. In these embodiments, an energy source such as a DCvoltage is applied between the two electrodes and electrons may begenerated from both the cathode (cold or hot) and the gas (coronadischarge) near the tip. The corona discharge is preferably minimized inorder to increase the efficiency of forming negatively charged hydrogenions by electron attachment and increase the lifetime of the cathode tipdue to a minimized collision of the positive ions on the cathodesurface.

In certain preferred embodiments of the cathode emission mechanismsdescribed above, the voltage applied across the two electrodes may beconstant or pulsed. The frequency of the voltage pulse ranges from 0 to100 kHz. FIGS. 1 a and 1 b provide an illustration of a voltage pulsefor a cathode and for an anode, respectively. In these embodiments, itis believed that a pulsed voltage may be preferable to a constantvoltage to improve the amount of electron emission and to reduce thetendency of gas phase discharge.

For embodiments involving electron generation through gas discharge,these embodiments may include thermal discharge, photo-discharge, andvarious avalanche discharge, including glow discharge, arc discharge,spark discharge, and corona discharge. In these embodiments, electronsare generated by gas phase ionization. In certain embodiments of gasphase ionization, the gas phase contains a reducing gas and an inertgas, a voltage source is applied between two electrodes and electronsare generated from the inert gas between the two electrodes that thendrift toward the positively biased electrode such as the anode. Duringthis electron drift, some of these electrons may attach on the reducinggas molecules and form negatively charged ions by electron attachment.In addition, some positive ions are also created by gas phase ionizationwhich then drift toward the negatively biased electrode such as thecathode and are neutralized at the electrode surface.

As mentioned previously, for cathode emission, the electrons are emittedfrom an electrode that may act as a cathode. Referring to FIGS. 2 athrough 2 i, the electrode may have a variety of geometries, such as,for example, a thin wire 2 a, a rod with a sharpened tip 2 b, a rod withseveral sharpened tips or comb 2 c, a screen or wire mesh 2 d, a loosecoil, an array of combs 2 f, a bundle of thin wires or filament 2 g, arod with sharp tips protruding from its surface 2 h, or a plate with aknurled surface 2 i. Additional geometries may include combinations ofthe above geometries such as plates or rods with surface protrusions,rods wrapped with wire windings or filament, coils of thin wires, etc. Aplurality of electrodes may be employed that may be arranged in aparallel series or in an intersecting grid. In certain embodiments, suchas embodiments wherein field emission is involved, the cathode ispreferably made of geometries having a large surface curvature, such asa plurality of sharp tips to maximize the electric field near theelectrode surface such as the geometry depicted in FIG. 3. As FIG. 3illustrates, electrode 1 has a series of thin wires 2 that reside withingrooves on the electrode surface along with a plurality of tips 3emanating from its surface.

The electrode material that acts as a cathode is preferably comprised ofa conductive material with relatively low electron-emission energy orwork function. The material preferably also has a high melting point andrelatively high stability under processing conditions. Examples ofsuitable materials include metals, alloys, semiconductors, and oxidescoated or deposited onto conductive substrates. Further examplesinclude, but are not limited to, tungsten, graphite, high temperaturealloys such as nickel chromium alloy, and metal oxides such as BaO andAl₂O₃ deposited onto a conductive substrate.

The electrode that acts as an anode is comprised of a conductivematerial such as a metal or any of the other materials describedtherein. The anode can have a variety of different geometries dependingupon the application such as any of the geometries described herein. Theanode can be grounded or connected to a voltage level with a positivebias relative to the cathode. To prevent neutralization of thenegatively charged ions at the anode, the anode may have an insulatinglayer such as a ceramic or glass layer atop the conductive material toprevent the neutralization of the negatively charged ion at the anode.

In alternative embodiment, the neutralization of negatively charged ionsto the anode surface may be minimized by applying a magnetic fieldduring electron generation. In this embodiment, a magnetic field may begenerated, such as for example, by providing one or more magnetic coilsor an external magnetic source either within or outside of the iongenerator apparatus and/or anode tube. Regardless of whether one or moremagnetic coils are outside or inside the anode tube, the electronmovement outside the tube may be driven by the electric field and theelectron movement inside the tube may be confined by the magnetic field.The strength of the magnetic field that is generated may vary butgenerally ranges from 0.1 to 5,000 Wb/m², or from 500 to 2,000 Wb/m². Incertain embodiments, a higher magnetic field may reduce the radius ofthe spiral path of the ion movement. In these embodiments, the radius ofthe ion spiral path may need to be smaller than the radius of the anodetube.

In certain embodiments of the present invention involving thermal-fieldemission, the cathode or emission electrode may comprise a segmentedassembly such as the electrode depicted in FIG. 4. In this regard, thecore 10 of the emission electrode may be made of a metal with a highelectric resistance and may have a plurality of tips 11 emanating fromcore 10. Tips 11 may be made of a conductive material with relativelylow electron emission energy or work function such as any of thematerials disclosed herein. The core may be heated by directly passingan energy source such as AC or DC current (not shown) through core 10.The thermal conduction will transfer the heat from the core to tips 11.The hot core may be enclosed within an insulating material 12 having aplurality of tips 11 exposed outside the enclosure which are theninserted into a support frame thereby forming a segmented assembly asshown. The segmented assembly may allow for the thermal expansion of thecore during operation. In this arrangement, electrons can be generatedfrom hot tips 11 by applying a voltage potential between the cathode andan anode.

In another preferred embodiments of the present invention involvingthermal-field emission, the temperature of the emission electrode can beraised by indirect heating. This may be accomplished by using a heatingcartridge as the core of the emission electrode. The surface of theheating cartridge may be comprised of an electric conductive materialsuch as a metal that is electrically insulated from the heating elementinside the cartridge. To promote electron emission, a plurality ofdistributed emission tips can be mounted on the surface of the heatingcartridge. The cartridge can be heated by passing an energy source suchas, for example, an AC or DC current through the heating element insidethe cartridge. Electrons can be emitted from the distributed tips of thecartridge by applying a negative voltage bias on the surface of thecartridge relative to a second electrode. For creating the voltage biasin this arrangement, the second electrode can be grounded so that thecartridge may be negatively biased or, alternatively, the cartridge canbe grounded so that the second electrode may be positively biased. Insome embodiments, the latter case may be preferable for eliminating apotential interference between two electric circuits, one is the AC orDC current along the heating element, and the another one is the highvoltage bias between the surface of the cartridge and the secondelectrode. In these embodiments, the hot cartridge electrode may alsoact as a heat source for the gas mixture to achieve the requiredtemperatures for reflow and soldering processes.

As mentioned previously, a gas mixture comprising a reducing gas ispassed through the remote ion generator containing at least twoelectrodes. The reducing gas contained within the gas mixture may fallwithin one or more of the following categories: 1) an intrinsicallyreductant gas, 2) a gas capable of generating active species which formgaseous oxides upon reaction of the active species with the metal oxide,or 3) a gas capable of generating active species which form liquid oraqueous oxides upon reaction of the active species with the metal oxide.

The first category of gases, or an intrinsically reductant gas, includesany gas that thermodynamically acts as a reductant to the oxides to beremoved. Examples of intrinsically reductant gases include H₂, CO, SiH₄,Si₂H₆, formic acid, alcohols such as, for example, methanol, ethanol,etc., and some acidic vapors having the following formula (III):

In formula (III), substituent R may be an alkyl group, substituted alkylgroup, an aryl, or substituted aryl group. The term “alkyl” as usedherein includes straight chain, branched, or cyclic alkyl groups,preferably containing from 1 to 20 carbon atoms, or more preferably from1 to 10 carbon atoms. This applies also to alkyl moieties contained inother groups such as haloalkyl, alkaryl, or aralkyl. The term“substituted alkyl” applies to alkyl moieties that have substituentsthat include heteroatoms such as O, N, S, or halogen atoms; OCH₃; OR(R=alkyl C₁₋₁₀ or aryl C₆₋₁₀); alkyl C₁₋₁₀ or aryl C₆₋₁₀; NO₂; SO₃R(R=alkyl C₁₋₁₀ or aryl C₆₋₁₀); or NR₂ (R═H, alkyl C₁₋₁₀ or aryl C₆₋₁₀).The term “halogen” as used herein includes fluorine, chlorine, bromine,and iodine. The term “aryl” as used herein includes six to twelve membercarbon rings having aromatic character. The term “substituted aryl” asused herein includes aryl rings having substitutents that includeheteroatoms such as O, N, S, or halogen atoms; OCH₃; OR (R=alkyl C₁₋₁₀or aryl C₆₋₁₀); alkyl C₁₋₁₀ or aryl C₆₋₁₀; NO₂; SO₃R (R=alkyl C₁₋₁₀ oraryl C₆₋₁₀); or NR₂ (R═H, alkyl C₁₋₁₀ or aryl C₆₋₁₀). In certainpreferred embodiments, the gas mixture contains hydrogen.

The second category of reducing gas includes any gas that is not anintrinsically reductive but can generate active species, such as, forexample, H, C, S, H′, C′, and S′, by dissociative attachment of electronon the gas molecules and form gaseous oxides by reaction of the activespecies with the metal oxides to be removed. Examples of this type ofgas include: NH₃, H₂S, C₁ to C₁₀ hydrocarbons such as but not limited toCH₄, C₂H₄, acidic vapors having the formula (III), and organic vaporshaving the following formula (IV).

In formulas (III) and (IV), substituent R may be an alkyl group,substituted alkyl group, an aryl, or substituted aryl group.

The third category of gas includes any gas that is not an intrinsicallyreductive but can form active species, such as, for example, F, Cl, F′,and Cl′, by dissociative attachment of electron on the gas molecules andform liquid or aqueous oxides by reaction of the active species with themetal oxides. Examples of this type of gas include fluorine and chlorinecontaining gases, such as CF₄, SF₆, CF₂Cl₂, HCl, BF₃, WF₆, UF₆, SiF₃,NF₃, CClF₃, and HF.

Besides including one or more of the above categories of reducing gases,the gas mixture may further contain one or more carrier gases. Thecarrier gas may be used, for example, to dilute the reducing gas ordiluting the reactive gas or provide collision stabilization. Thecarrier gas used in the gas mixture may be any gas with an electronaffinity less than that of the reducing gas within the gas mixture. Incertain preferred embodiments, the carrier gas is an inert gas. Examplesof suitable inert gases include, but are not limited to, N₂, Ar, He, Ne,Kr, Xe, and Rn.

In certain preferred embodiments, the gas mixture comprises hydrogen asthe reducing gas and nitrogen as the carrier gas due to its relativelylower cost and the environmental friendliness of the exhaust gasrelease. In these embodiments, the gas mixture comprises from 0.1 to100% by volume, preferably 1 to 50% by volume, or more preferably from0.1 to 4% by volume of hydrogen. Amounts of hydrogen lower than 4% arepreferred, which makes gas mixture non-flammable.

In certain embodiments, the gas mixture is passed through the iongenerator at a temperature ranging from ambient to 3500° C., or rangingfrom 150 to 1500° C. for forming active species. After passing throughthe ion generator, the gas mixture may then be reduced to processingtemperatures, such as reflow and soldering temperatures, for surfacede-oxidation. The pressure of the ion generator may range from 1 to 20atmospheres, or from 1 to 5 atmospheres. The pressure of the negativelycharged ionic reducing gas at the outlet of the ion generator can bereduced to 1 atmosphere before entering the furnace or treatment area byusing, for example, a flow restricting orifice, backpressure regulator,flow controller, or similar means.

As mentioned previously, the component or work piece in which the oxideis to be removed and/or soldered is preferably disposed within closeproximity to the gas outlet of the ion generator. The distance betweenthe outlet and the top surface of the component may range from 0.1 to 30cm, or from 0.5 to 5 cm. In certain preferred embodiments of the presentinvention, the component may be disposed upon a substrate to provide atarget assembly. The substrate may be grounded or, alternatively, have apositively biased electrical potential. In an alternative embodiment,the component may be interposed between the outlet of the ion generatorand substrate that is grounded or has a positively biased electricalpotential.

In certain embodiments, the remote ion generator and/or the component(or target assembly) may be moved. In this regard, the remote iongenerator may be in a fixed position and the component may be moved, theremote ion generator may be moved and the component may be in a fixedposition, or both the remote ion generator and the component are moved.The movement may be vertical, horizontal, rotational, or along an arc.

FIG. 5 provides an illustration of one embodiment of the presentinvention used, for example, in reflow soldering. The apparatuscomprises an oven or furnace 20 which may typically have heating/coolingzones located at different sections along the center axis of theoven/furnace 20. A remote ion generator 21 having at least twoelectrodes (not shown) is inserted within the interior of oven/furnace20. Ion generator 21 further has a gas inlet 22 and a gas outlet 23. Agrounded moving belt 24 made of a conductive material, such as a metal,carries one or more components 25, such as electronic devices on aprinted circuit board which are temporarily connected together by solderpaste previously printed on one or more of the components, which passthrough the oven/furnace 20 and the heating and cooling zones. A gasmixture 26 comprised of nitrogen and a hydrogen reducing gas isintroduced into ion generator 21 through gas inlet 22, and an energysource (not shown) is applied to at least of the two electrodes actingas a cathode and an anode contained therein. The gas mixture 26 respondsto the charge between the cathode and the anode by electrons beinggenerated at the site of the cathode to the reducing gas, preferablyhydrogen, to become a negatively charged ionic reducing gas 27 whichpasses through gas outlet 23. Gas outlet 23 is in close proximity tocomponents 25. The negatively charged ionic reducing gas 27 reduces anyexisting metal oxides on the surface of component and solder therebysignificantly enhancing solder joining. The solder paste is melted in aheated zone of the oven/furnace 20, wets the surface of the componentsand resolidifies in the cool zone of the oven/furnace 20 to form thesoldered product, which requires no flux and avoids solder imperfectionscaused by oxides or flux residues.

FIG. 6 provides an example of one embodiment of the remote ion generatorapparatus 30 of the present invention. Ion generator 30 comprises atleast two electrodes: a cathode 31 having a coil geometry and a metalanode 32 which comprises the walls of the apparatus. Cathode 31 andanode 32 are connected to an external energy source (not shown). Metalanode 32 further comprises a ceramic liner 33 which is disposed upon itssurface as shown. Ion generator 30 further has a gas inlet 34 and gasoutlet 35. The geometries of gas inlet 34 and 35 can vary with respectto each other to affect the flow velocity of the gas mixture (not shown)which passes through ion generator 30. A gas mixture (not shown)containing a reducing gas and optionally a carrier gas is passed throughion generator 30. An energy source (not shown) such as DC voltage ispassed through cathode 31 and anode 32 thereby generating an electricfield and causing cathode 31 to generate electrons. The electronsgenerated from cathode 31 drift in the direction of the electric field.The electrons attach to at least a portion of the reducing gas therebygenerating a negatively charged reducing ionic gas (not shown). Thenegatively charged reducing ionic gas passes through the gas outlet 35to reduce the surface oxides of the component (not shown).

FIG. 7 provides an illustration of another embodiment of the remote iongenerator of the present invention. Ion generator 40 has two gas inlets41 and 42 that flow into two concentric chambers, 43 and 44, to ensuresafety for treating a concentrated reducing gas such as hydrogen. Theconcentrated hydrogen gas enters through gas inlet 42 into main chamber43. Main chamber 43 is surrounded with a secondary chamber 44 that ispurged with a carrier gas such as nitrogen that enters via inlet 41. Thepressure of the concentrated hydrogen stream is maintained to be higherthan that of nitrogen in the secondary chamber 44 and the ratio of thegas flow rates between the concentrated hydrogen stream and the nitrogenstream is adjusted to a level that the total concentration of hydrogenin the mixture of the two streams is equal to or less than 4% by volume.

FIG. 10 provides an example of an ion generation apparatus 50 thatfurther includes a magnetic coil 51. While apparatus 50 shows magneticcoil 51 as the source for the magnetic field, it is anticipated thatother sources for generating a magnetic field besides a coil areanticipated herein. Magnetic coil 51 can reside within the internalvolume anode 54 as shown, reside outside or upon anode 54, or a varietyof other configurations. Ion generator 50 comprises at least twoelectrodes: a cathode assembly having a “comb-shaped” cathode emitter 52(such as the electrode configuration shown in FIG. 2 c) and is disposedwithin cathode holder 53 and anode 54 which comprises the walls of theapparatus. Cathode emitter 52 is comprised of a conductive material suchas any of the materials disclosed herein. Cathode holder 53 is comprisedof an insulating material. However, in alternative embodiments, cathodeholder 53 can be comprised of a conductive material, an insulatingmaterial, a semiconductive material or a combination thereof. In certainembodiments, the tips of cathode emitter 58 can extend into the internalvolume within anode 54 through a plurality of perforations 55 within thewalls of anode 54. Alternatively, tips 58 can reside outside theinternal volume within anode 54, such as the area between cathodeemitter and anode 54.

Cathode emitter 52, anode 54, and magnetic coil 51 are connected to anexternal energy source as shown. In certain embodiments, metal anode 54may be comprised of one or more layers of conductive material, aninsulating material that is at least partially coated with a conductivematerial, or a variety of other configurations. Ion generator 50 furtherhas a gas inlet 56 and gas outlet 57. The geometries of gas inlet 56 and57 can vary with respect to each other to affect the flow velocity ofthe gas mixture (not shown) which passes through ion generator 50. A gasmixture (not shown) containing a reducing gas and optionally a carriergas is passed through ion generator 50. An energy source such as DCvoltage is applied between cathode emitter 52 and anode 54 therebygenerating an electric field and causing cathode emitter 52 to generateelectrons. The magnetic field that is generated by applying electriccurrent flow through coil 51 or other magnetic source may aid indirecting the electrons out of ion generator 50 through gas outlet 57.Parameters such as, but not limited to, the size and number ofperforations 55 on anode 54, the number of windings on magnetic coil 51,the configuration, number, and angle of the conductive tips 58 oncathode emitter 52, the gap between the tips of the cathode emitter 52and surface of anode 54, the voltage between the two electrodes, thecurrent within magnetic coil 51 and the geometry of anode 54, can bevaried to effect the amount of electrons generated.

The method disclosed herein can be used in several areas of theelectronic assembly besides soldering such as, for example, surfacecleaning, metal plating, brazing, welding, and reflow of a solder bumpedwafer. In one embodiment, the present invention is used for the reflowof a solder bumped wafer such as the method provided in copending U.S.application Ser. No. 10/425,405, filed Apr. 28, 2003, which is assignedto the assignee of the present invention. In one particular embodiment,the method can be used to reduce surface oxides of metals, such ascopper oxide, formed during silicon wafer processing. Such oxides mayform as a result of the various wet processing steps, such as chemicalmechanical planarization, that are used to form micro-electronic deviceson the wafers. These surface oxides reduce device yield and devicereliability. The method allows surface oxides to be removed in a fullydry, environmentally friendly manner that does not require the use ofaqueous reducing agents. Further, since the method can be performed atrelatively low temperatures, it does not significantly affect thethermal budget of the device during processing. Higher temperatures, bycontrast, tend to reduce device yield and reliability by causingdiffusion of dopants and oxides thereby reducing device performance.Since the method can also be performed on a single wafer, the method canbe integrated with other single wafer processes, thereby providingbetter compatibility with other fabrication steps.

The invention will be illustrated in more detail with reference to thefollowing examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLE 1

A first experiment was conducted by using a lab-scale, tube furnacehaving a downward-facing metal rod with a sharp tip (see FIG. 2 b forcathode geometry) inserted near the center of the furnace. The sampleused was a fluxless tin-lead solder preform (m.p. 183° C.) on a groundedcopper plate (anode), which was loaded inside a furnace and heated up to250° C. under a gas flow of 5% H₂ in N₂. When the sample temperature wasat equilibrium, a DC voltage was applied between the negative electrode(cathode) and the grounded sample (anode) and gradually increased toabout −2 kV with a current of 0.3 mA. The distance between the twoelectrodes was about 1 cm. The pressure was ambient, atmosphericpressure.

It was found that the solder was well wetted on the copper surface.Without applying an electric voltage, a good wetting of a fluxlesssolder on a copper surface can never be achieved at such lowtemperature, even in pure H₂, because the effective temperature for pureH₂ to remove tin-oxides on a tin-based solder is above 350° C.Therefore, this result confirms that the electron-attachment method iseffective in promoting H₂ fluxless soldering.

It was proved in the small-scale test that the negatively chargedhydrogen ions are much more reactive than neutral hydrogen gasmolecules. Therefore, by using this new approach, the hydrogen reductionof oxides on both solder and base metal can be largely promoted. Theeffective temperature of hydrogen fluxless soldering (5 vol. % H₂ in N₂)under ambient pressure is reduced into the normal soldering temperaturerange (<250° C.).

EXAMPLE 2

Several cathode materials were investigated for electron-attachmentassisted hydrogen fluxless soldering by using the field emissionmechanism using the same set-up as Example 1. The results of theinvestigation is provided in Table I.

As Table I illustrates, the best result was obtained by using Ni/Crcathode, which provided the highest fluxing efficiency and thus resultedin the shortest wetting time. A possible reason is that the Ni/Crcathode generates a relatively larger quantity of electrons and has asuitable energy level of electrons compared to other cathode materials.

TABLE I Effect of Cathode Material on Wetting Time at 250° C. and 20% H₂Material of Cathode Rod With a Sharp Tip ( 1/16″ dia.) Time to CompleteWetting Brass 1 min 55 sec Copper 1 min 44 sec Nickel Chromium 39 secAluminum 1 min 28 sec Stainless Steal 1 min Tungsten 1 min 54 sec

EXAMPLE 3

In order to demonstrate the feasibility of the electron-attachmentassisted H₂ fluxless soldering using a remote ion generator, anexperiment was conducted wherein a remote ion generator was made using adownward-facing Ni/Cr cathode rod with a sharp tip. The anode consistedof a copper plate covered by a ceramic layer. The electric field appliedbetween the two electrodes was about 2 KV/cm. The remote ion generatorwas set in front of a test sample, or in other words, toward the gasinlet of the furnace. The distance between the remote ion generator andthe test sample was about 2 to 4 cm. The test sample consisting of aSn/Pb solder perform (m.p. 183° C.) on a copper plate was grounded. Whenthe furnace loaded with the remote ion generator and the test sample waspurged with a gas mixture of H₂ and N₂ (˜5% H₂ by volume) and heated up,it was found that at around 220° C., the solder started to wet on thecopper.

EXAMPLE 4

The concept of using a hot cathode together with an electric field(thermal-field emission) to promote the efficiency of electron emissionwas experimentally demonstrated using a thin Ni/Cr wire (0.004″diameter) that was hung down the center of a vertically oriented tubefurnace. The wire was heated by an AC power and connected to thenegative side of a DC power source to provide a hot cathode. In the samefurnace, a grounded metal plate was also hung in parallel to the hotwire to provide an anode. The gap between the anode and cathode was 1.43cm. The cathode temperature was measured at various temperatures rangingfrom room temperature, or ambient, to 650° C. using a thermocouple incontact with the cathode surface. During the experiments, a nitrogenflow was maintained in the furnace and electrons were generated from thehot cathode.

The emission current at different cathode temperatures as a function ofthe applied DC voltages between the two electrodes is provided in FIG.8. FIG. 8 illustrates that the cathode emission current is significantlyincreased when the cathode temperature is increased from ambienttemperature to above 200° C. The greatest increase in emission currentoccurred below 400° C.

EXAMPLE 5

The present example was conducted to investigate the effectiveness ofthe thermal-field emission method for generating electrons. A 3 mmdiameter graphite rod, having a number of 1 mm long machined tipsprotruding from its surface, acted as the cathode and had a geometrysimilar to that depicted in FIG. 2 h. Each of the protruding machinedtips had a tip angle of 25 degrees. The graphite rod was heated up in agas mixture of 5% H₂ and 95% N₂ to about 400 to 500° C. by resistiveheating using an AC power source. A DC voltage source of 5 KV wasapplied between the graphite cathode and an copper plate that acted asan anode having a 1.5 cm gap there between. All the tips on the graphiterod were illuminated thereby indicating that electrons could uniformlybe generated from the distributed tips on the graphite rod. Withoutheating of the graphite rod, there would be either no electrongeneration from the cathode, or arcing between one of the tips and theanode plate. This demonstrates that the combination of using a cathodehaving multiple tips and elevated temperatures, i.e., a thermal-fieldemission method, is effective for obtaining uniform electron generationfrom an integrated emitting system.

EXAMPLE 6

The present example was conducted using a 0.04″ diameter nickel-chromiumalloy heating wire clamped horizontally between two machined Al₂O₃refractory plates such as the electrode illustrated in FIG. 4. A seriesof five nickel-chromium alloy emitting wires, each with a sharp tip(12.5 degree) on one end of the wire, protruded perpendicularly from thenickel-chromium heating wire and were vertically positioned between tworefractory plates. The nickel-chromium heating wire and tips were heatedup in a gas mixture of 5% H₂ and 95% N₂ to about 870° C. using an ACpower source. A DC voltage of 2.6 KV was applied between the cathode anda copper plate that acted as the anode having a 6 mm gap between the twoelectrodes. All five tips were illuminated and the total currentemission reached 2.4 mA. Without heating of the wire, there would beeither no electron generation from the cathode, or arcing between one ofthe tips and the anode plate. Like example 5, example 6 demonstratesthat thermal-field emission provides uniform electron generation.Further, because of the higher temperature of the cathode, it alsoincreases the quantity of the electron generation at a given electricpotential.

EXAMPLE 7

The present example demonstrated the performance of a hot emissionelectrode with multiple-tips on the surface where the high temperatureof the electrode is achieved by indirect heating and the voltage appliedbetween the two electrodes is pulsed. The emission electrode had aheating cartridge as its core. The enclosure of the cartridge is made ofstainless steel which is electrically insulated with a heating elementinside the enclosure. Six nickel/chromium wires each having two sharptips (12.5 degree in tip angle) that extended at an angle of 90 degreesfrom each other were inserted into distributed grooves on the surface ofthe cartridge such as that depicted in FIG. 3. The cartridge was heatedto a temperature of 800° C. in an atmosphere containing 5% H₂ and 95% N₂by passing a AC current through the heating element. A pulsed DC voltagesource was applied between the surface of the emission electrode and agrounded copper electrode. The gap between the two electrodes is about 1cm.

FIG. 9 illustrates that the emission current increases when theamplitude and the frequency of the voltage pulsing is increased.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A method of chemically reducing metal oxides to base metals on thesurface of an at least one component comprising said metal oxides, themethod comprising the steps of: providing the at least one componentwhich is connected to a substrate as a target assembly wherein thesubstrate has at least one electrical potential selected from the groupconsisting of a grounded or a positive electrical potential; passing agas mixture comprising a reducing gas through a remote ion generatorcomprising a gas inlet, a gas outlet in fluid communication with the gasinlet, an anode, a cathode, and a magnetic coil wherein the magneticcoil is interposed between the gas inlet and the gas outlet wherein thegas outlet is proximal to the target assembly; supplying energy to atleast one of the cathode and the anode to generate electrons that attachto at least a portion of the reducing gas passing through the iongenerator thereby forming a negatively charged reducing gas at the gasoutlet; supplying energy to the magnetic coil to provide a magneticfield proximal to the negatively charged reducing gas; and contactingthe target assembly with the negatively charged reducing gas to reducethe metal oxides on the at least one surface of the at least onecomponent, wherein the pressure within the remote ion generator is from1 to 20 atmospheres.
 2. The method of claim 1 wherein the anodecomprises an insulating material disposed upon at least a portion of itssurface.
 3. The method of claim 1 wherein the temperature of the gasmixture within the ion generator ranges from 25° C. to 3,500° C.
 4. Themethod of claim 3 wherein the temperature of the gas mixture within theion generator ranges from 150° C. to 1,500° C.
 5. The method of claim 1wherein the temperature of the cathode ranges from 25° C. to 3,500° C.6. The method of claim 5 wherein the temperature of the cathode rangesfrom 150° C. to 1,500° C.
 7. The method of claim 1 wherein the energy inthe supplying step is at least one source selected from the groupconsisting of an electric energy source, an electromagnetic energysource, a thermal energy source, a photo energy source, and combinationsthereof.
 8. The method of claim 1 wherein the energy is applied to thecathode.
 9. The method of claim 1 wherein the pressure within the iongenerator ranges from 1 to 5 atmospheres.
 10. The method of claim 1wherein the cathode is composed of a material selected from the groupconsisting of brass, stainless steel, copper, nickel chromium, aluminum,tungsten, graphite, metal oxides deposited on a metal substrate, andmixtures thereof.
 11. The method of claim 10 wherein the cathode iscomposed of nickel chromium.
 12. The method of claim 1 wherein thecathode has a geometry selected from the group consisting of a wire, acoil, a screen, a rod, a rod with a sharp tip, an array of rods withsharp tips, a brush comprised of wires, a plate with protrusionsemanating from at least one of its surfaces, a rod with protrusionsemanating from its surface, and mixtures thereof.
 13. The method ofclaim 1 wherein the distance between the component and the outlet of theion generator ranges from 0.1 to 30 cm.
 14. The method of claim 13wherein the distance between the component and the outlet of the iongenerator ranges from 0.5 to 5 cm.
 15. The method of claim 1 wherein theremote ion generator is moved.
 16. The method of claim 1 wherein thetarget assembly is moved.
 17. The method of claim 16 wherein the remoteion generator is moved.
 18. The method of claim 1 wherein the method isused in at least one process from the group consisting of reflowsoldering, wave soldering, wafer bumping, metal plating, brazing,welding, surface cleaning, thin film de-oxidation, and mixtures thereof.19. The method of claim 1 wherein the substrate has a positiveelectrical potential.
 20. The method of claim 1 wherein the substrate isgrounded.
 21. The method of claim 1 wherein the reducing gas is selectedfrom the group consisting of H₂, CO, SiH₄, Si₂H₆, CF₄, SF₆, CF₂Cl₂, HCl,BF₃, WF₆, UF₆, SiF₃, NF₃, CClF₃, HF, NH₃, H₂S, straight, branched orcyclic C₁ to C₁₀ hydrocarbons, formic acid, alcohols, acidic vaporshaving the following formula (III):

organic vapors having the following formula (IV),

and mixtures thereof wherein substituent R in formula III and IV is analkyl group, a substituted alkyl group, an aryl group, or a substitutedaryl group.
 22. The method of claim 21 wherein said gas mixturecomprises from 0.1 to 100% by volume of hydrogen.
 23. The method ofclaim 22 wherein said gas mixture comprises from 0.1 to 4% by volume ofhydrogen.